From 70565f2be8807e5ea24dfb421197b881a02af5e2 Mon Sep 17 00:00:00 2001 From: Barry Song Date: Thu, 5 Sep 2024 20:11:24 +1200 Subject: [PATCH] mm: arm64: document why pte is not advanced in contpte_ptep_set_access_flags() According to David and Ryan, there isn't a bug here, even though we don't advance the PTE entry, because __ptep_set_access_flags() only uses the access flags from the entry. However, we always check pte_same(pte, entry) using the first entry in __ptep_set_access_flags(). This means that the checks from 1 to nr - 1 are not comparing the same PTE indexes (thus, they always return false), which can be a bit confusing. To clarify the code, let's add some comments. Reviewed-by: Ryan Roberts Signed-off-by: Barry Song Cc: Ard Biesheuvel Cc: John Hubbard Cc: Mark Rutland Cc: Catalin Marinas Cc: David Hildenbrand Cc: Will Deacon Reviewed-by: David Hildenbrand Link: https://lore.kernel.org/r/20240905081124.9576-1-21cnbao@gmail.com Signed-off-by: Will Deacon --- arch/arm64/mm/contpte.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index a3edced29ac1..55107d27d3f8 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -421,6 +421,12 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma, ptep = contpte_align_down(ptep); start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); + /* + * We are not advancing entry because __ptep_set_access_flags() + * only consumes access flags from entry. And since we have checked + * for the whole contpte block and returned early, pte_same() + * within __ptep_set_access_flags() is likely false. + */ for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) __ptep_set_access_flags(vma, addr, ptep, entry, 0);